This invention pertains to controllers utilizing reference generators and tachometers for controlling acceleration and speed of induction motors, and particularly to systems for controlling not only acceleration and speed but also for controlling stopping of motors and loads by induction.
The present controller like prior controllers utilizes thyristors, reference or ramp generators, and tachometers to control rate of acceleration and speed of three-phase induction motors. In a typical three-phase, 240-volt system, each conductor of a three-phase line is connected through a respective thyristor and diode to the winding of an induction motor. Firing circuits for determining conductive intervals of the thyristors are controlled by an error signal from an output of a comparator. Inputs to the comparator include a voltage proportional to speed derived from a tachometer connected to the motor and a voltage having a predetermined rate of change derived from a reference generator. The reference generator is known as a ramp generator for a curve of its output voltage or ramp is substantially linear with a predetermined slope determining acceleration. The motor is accelerated at a rate such that the output from the tachometer to the comparator is maintained substantially equal to the instantaneous voltage of the ramp.
Since in many applications of motors having controlled acceleration, controlled deceleration is not required, motors and loads coast to a stop at a rate determined by friction. In other applications for operating such devices as passenger elevators, deceleration and stopping must be smoothly controlled. Mechanical brakes are often used even though large amounts of energy dissipated by the brakes result in undesirable amounts of wear.
Multi-speed, three-phase, induction motors have been used for electromagnetic braking by changing connections to windings in two different ways. To provide braking from high speed operation to low speed operation, the conductors of a supply line to windings are merely changed to low speed operation, and the motor functions as a generator to provide braking. However, current flow during this regenerative braking must be controlled to prevent excessive current and a high rate of deceleration. For stopping the motor and its load, two connections to the windings are interchanged so that the rotor of the motor is urged in a reverse direction. The result of this interchange is called plug-reverse braking and because it is very effective requires current control to prevent abrupt stopping and also to prevent damaging high surges of current.
During plug-reverse braking, the phase of lagging current in the winding of the motor is nearly 90 degrees in an adjacent quadrant with respect to the phase of voltage applied to the winding. During this period of lagging current while the motor is functioning as a generator, the thyristor for the particular line continues firing for a substantial period after the zero crossings of the applied voltage would ordinarily, when current is in phase, have reversed the polarity on the terminals of the thyristor to stop firing. Even though the firing of a thyristor is started only shortly before the zero crossing, the amount of lagging current in the winding is excessive and causes elevators to stop abruptly in a manner known as jerking. Also when using plug-reverse braking, current must be controlled accurately to prevent overheating the motors.
When motors to be controlled have high starting torque and the starting current lags nearly 90 degrees, severe jerking while starting can be experienced. The present delayed firing is very effective to provide gradual starting.